In commonly owned U.S. Pat. No. 3,117,348 to Herbert Rees there has been disclosed an injection-molding machine with a first mold portion on a movable platen and a second mold portion on a quasi-stationary platen which is supported by a spring mounting on a fixed backing plate, the latter forming one anchor for a set of tie bars whose other anchor is a superstructure on the machine bed containing a mechanical clamping unit. An injection unit rigid with the fixed backing plate has an orifice leading to a sprue in the substantially stationary mold portion, this orifice being blocked in the open-mold position by a valve which is repressed by that mold portion upon displacement thereof by the movable portion so that molten plastic material may pass from a precompression chamber (also known as a shooting pot) through that orifice and the sprue into a cavity. This occurs in the terminal phase of the mold-closing stroke in which the cavity-forming mold portion moves together with its supporting platen against the countervailing spring pressure toward the backing plate, the platen then coming to rest against that plate.
With a hydraulic mold drive operating in two stages, i.e. under relatively low pressure during the major part of a mold-closing stroke to establish contact between the mold portions and with an increased clamping pressure exerted in the final phase of that stroke, a floating platen limitedly movable on a fixed backing plate (as disclosed in the above-identified Rees patent) would not be useful. In such a machine the terminal clamping pressure is absorbed by an elastic stretch of the tie bars to which the quasi-stationary platen is attached; this platen, therefore, should have limited mobility on the machine bed to allow a certain extension of the tie bars, generally on the order of one millimeter. The displacement of this platen is consequently substantially shorter than in the conventional case, owing to the higher modulus of elasticity of the tie bars compared with the countervailing springs of the Rees patent. Because, moreover, the clamping pressure is relatively low at the beginning of the tie-bar stretch, it is no longer possible to open the injection valve upon initial contact between the mold portions since this would allow the precompressed plastic mass to leak out at their junction and to cause flashing so as to spoil the product.
A possible solution to this problem resides in keeping the injection unit retracted from the quasi-stationary platen and advancing it into a valve-opening position only in response to a signal showing that the mold has been fully closed. This, however, would cause a significant delay in the start of the molding operation with objectionable lengthening of the work cycle.
Conceivably, the advance of the injection unit could be timed to occur slightly earlier so that the valve member meets the quasi-stationary mold portion just at the end of the closing stroke. Such a solution is also not very practical because of the short time involved whereby variations in oil viscosity and other operating parameters may again cause a premature opening of the injection orifice.